1. Field of the Invention
The present invention relates to a magneto-optical recording medium on and from which information is optically recorded and reproduced using a light beam, and a method of recording information on the medium.
2. Related Background Art
As conventional media used for optically recording/reproducing information, a ROM (read-only) type medium, a WORM (write once, read many) type medium, an R/W (rewritable) type medium, and the like are known. These media use a transparent material, such as glass or polycarbonate, as a substrate, and are distinguished from each other depending on the material used to form a coating or a film on the substrate. That is, when a substance such as Al having high reflectance and thermal stability is used on the substrate, a ROM type medium is obtained. When a material, such as an organic dye that causes an irreversible reaction due to heat is used, a WORM type medium is obtained. Also, when a material such as a magnetic material or a phase change material (which can assume both crystalline and amorphous states), which can thermally or magnetically cause a reversible reaction, is used, an R/W type medium is obtained.
On the other hand, when optical information recording media are classified in terms of their shapes, they can be roughly classified into a disc type medium, a card type medium, and a tape type medium. These media have respective features and are selectively used depending on application. Of these types of media, the disc type medium is most popular since it can realize high-speed information transfer.
FIG. 1 shows an example of a magneto-optical recording medium as a conventional R/W type optical disc. The medium mainly comprises a first magnetic layer 3 (to be referred to as a reproduction layer hereinafter), an intermediate layer 2, and a second magnetic layer 5 (to be referred to as a memory layer hereinafter). The memory layer 5 is a film such as TbFeCo, DyFeCo, or the like having a large perpendicular magnetic anisotropy, and recording information is held by forming magnetic domains depending on whether the direction of magnetization of this film is upward or downward with respect to the film surface. The reproduction layer 3 is a film, such as GdFeCo, having a small coercive force and a high Curie temperature. The intermediate layer 4 consists of a dielectric, such as SiN, and is arranged to magnetostatically couple the memory layer 5 and the reproduction layer 3. Information reproduction is attained by detecting magnetic domains transferred from the memory layer 5 to the reproduction layer 3 by the magnetostatic coupling force using a reading laser beam. Even when the reproduction layer and the intermediate layer are omitted (the memory layer alone), information may be recorded/reproduced. However, in this case, high recording sensitivity and high signal quality are hardly accomplished at the same time. More specifically, in the above-mentioned prior art, a material that allows information recording with low power is selected for the memory layer 5, and a material that allows information reproduction with a high CNR is selected for the reproduction layer 3.
However, in the above-mentioned prior art, when the memory layer and the reproduction layer are magnetostatically coupled to each other, different magnetostatic forces are generated depending on the sizes of recording marks. This problem will be explained below with reference to FIGS. 2A to 2C.
FIG. 2C shows the calculation results of the magnitudes, H.sub.st, of static magnetic fields generated by a circular magnetic domain and a rectangular magnetic domain. A graph (a) represents the strength of the static magnetic field 30 nm above a cylindrical magnetic domain having a diameter of 0.4 .mu.m, a thickness of 30 nm, and a saturation magnetization of 200 emu/cc. As can be seen from the graph (a), a static magnetic field of about 300 Oe is present near the center of the magnetic domain although it has a peak near the magnetic wall. In contrast to this, in the case of a rectangular parallelopiped magnetic domain having a size of 1.6.times.0.8 .mu.m, a thickness of 30 nm, and a saturation magnetization of 200 emu/cc, as shown in a graph (b), the static magnetic field generated is weak as a whole and only a static magnetic field as low as about 100 Oe acts near the center of the magnetic domain. For this reason, the transfer characteristics of magnetic domains from the memory layer to the reproduction layer are impaired, and stability against an external magnetic field is low.